Monday, May 8, 2017

In a Drought, July Showers Bring May Calling

In 1992-93, as concern was escalating about the global
decline of Amphibians, we established a system of two auditory monitoring
transects north and south from our home in Bishops Mills. This system of 42
stations, plus 'backyard' listening from home,
extends 51 km NNE from just north of Brockville to the north shore of
the Rideau River. As the years went by,
it has acquired the moniker “More than one Person can handle,” and with the
onrush of other spring-time and summer duties we haven't often been able to
listen adequately along the whole transect.
Because of a road we didn't know was impassable when we planned the
transects from the topo map, the system has broken down into 3 transects, one
to the south, one to the north, and one, most frequently surveyed, around home.
When we are home, we also listen from home every night, when it's not
excessively rainy or windy, from 1 March to 1 August.

Three appropriately timed surveys on
nights with appropriate weather can get good records of all Anuran species
calling in eastern Ontario, but one of the lessons we've learned, in what's now
34 years of experience, is that good seasons for frogs are often bad for
auditory monitoring. The best seasons for listening to spring-calling species
are those in which the temperature warms gradually, with modest precipitation
and moderate week-long oscillations in temperature. On the other hand, the best
seasons for these frogs are those in which warming and wetness coincide, to
bring the all the females to the ponds in a single wave, so mating is concluded
promptly, and both sexes spend a minimum of time and energy reserves in
completing the breeding, with maximum survival to breed again in a subsequent
year. This scenario minimizes the duration of calling, and severely constrains
the time available for auditory monitoring.

On the other hand, persistently dry seasons can be bad for
both frogs and monitors, since choruses are late in forming if small species,
particularly, won't move across dry ground to reach breeding ponds, many of the
frogs may desiccate on the way to the ponds, peak calling may occur on very different
dates at different sites, and some ponds may dry up during the calling season. Similar
but less easily understood factors seem to influence calling by Mink/Green/Bull
Frogs; one that is fairly easy to grasp is that calling is prolonged, for Green
& Bull frogs at least, during dry
seasons, in which it seems that males continue to call after breeding is
finished to defend territories for feeding, whereas in wet summers they abandon
the wetlands for terrestrial foraging (hypothesis 3 of Schueler 2000).

In 2016 maps of conditions show record low precipitation
in the area of the surveys, and “In Ottawa, counting both snow and rainfall for
[1April to 28 July], the area has received only a little over half the normal
amount seen there from April through July. Based on Environment Canada weather
records, Ottawa International Airport recording its 2nd lowest precipitation
total for the growing season... since record keeping began there in 1938,
second only to the same period in 1955.”[1]This season we had support from the OFNC
Research Fund for driving the transect, which was an incentive to push other
obligations aside and go out frequently. A problem in listening along a
transect which you can't monitor every night is deciding on which nights you're
going run the transect, since you're trying to hear all the species that can be
heard from each station, and calling intensity, especially at stations where
the calling is from distant sites, is diminished by low temperatures and
changed by moisture levels.

Methodology:At each station we follow our 1994 auditory monitoring protocol: “Record
the time you listened, air temperature, wind and precipitation, sources of
distracting noise, and species that were calling (or that none were heard)...
we record all anurans, birds and mammals that we hear and all amphibians and
reptiles we see while we are listening. Intensity of calling is indicated by
the Wisconsin calling index (Index One = Individuals can be counted; no
overlapping of calls. Index Two = Calls of individuals are distinguished, but
some calls overlap. Index Three = Full chorus; continuous calls) or by counts
of individuals. We listen at a station until we are satisfied that we have
heard all calling anuran species, usually 2-5 minutes, but sometimes as long as
10 minutes if there is a lot of noise. We record starting and finishing times of
the visit; air temperature and wind, sky and distracting noise are described at
each stop.” (page 153, Karstad, et al. 1995).

Because of the uncertainty of when to go out when
conditions were so dry, and continually waiting for rain, we had a hard time deciding
when to go out through May, because the nights were cold and the days not warm,
and there were no rainy nights. Through
June there were 6 days when there was some rain, and it rained a bit on 2 July,
but the plants were transpiring heartily, and on 8 July the roadside ditch just
south of Bishops Mills, was crisply dry, with herbs on the floor of the ditch
all wilting. The night before I'd run the nearby transect, and had heard only
relatively unenthusiastic Bull, Green, and Mink Frogs. That night (22h19,22h36,
20.7°C, overcast, Beaufort gentle-light breeze) a few Bull Frog calls were
audible from home, over radio noise from the neighbour's barn (the radio is run
to scare Pigeons, Columba livia, away from the barn).

In the wee hours of 9 July, 02h30-02h40, we had the onset of forecast rain with thunder. This continued until about 4 cm fallen (29.2 mm total at the Environment Canada rain gauge in Kemptville, half of the rain measured there in all of July), and then on and off all day. At 15h18-15h26, as I “did the streets” in the village for on-road creatures, it was 20°C, calm, with light rain and thunder, and there were fresh skins of three Green Frogs and a Toad dead on the streets. The creek had about twice the minimal flow it had had the day before, and formerly dry garden soil was moist to a depth of 30 cm.

After dark, there was a moderate surrounding chorus of
Treefrogs, heard over radio & traffic noise in a trace of rain. Rain continued through the night with
juvenile Green Frogs (8 alive on road, 5 dead on road), Treefrogs (2 AOR, 2
DOR), 1 DOR juvenile Storeria occipitomaculata
(Redbelly Snake), and 3 Cepaea nemoralis
snails active on the sidewalk. On the next day there were 8 DOR and 3 AOR
juvenile Green Frogs, and 3 DOR and 1 AOR yearling Treefrogs on the streets.

Having heard Treefrogs and Toads from home on 10 July, I
set out to repeat the listening I'd done on 7 July. The results are
presented in the Table below. On this nearby circuit, 14 stations are
visited, at temperature of 20-18°C (22h47-25h02) on 7 July, & 14-12°C
(22h45-25h02) on 9 July, clear and calm on both nights. The time of visits
overlapped or was adjacent minutes at 10 stations, and was less than 10 minutes
different at the others, so comparisons between the nights don't reflect
changes in intensity of calling through the nights (this precise temporal
matching was purely fortuitous, and was not planned). Air temperatures varied
in parallel among stations, and was 6°C cooler on 10 July at 8 stations, 5°C
cooler at 4, and 7°C cooler at two.

The most widespread change after the rain was reduced
calling by Green and Bull frogs (reduced scores for Bull Frogs at 9 stations,
for Green Frogs at 5; Green Frogs not heard where they had been heard on 7 July
at 6 stations, Bull Frogs at 1; Green Frogs
heard where they had not been heard on 7 July at 1 station). The closely related Mink Frog was heard at
two stations where it had not been heard on 7 July.

Spring-calling species which were only heard after the
rain were Treefrogs (5 stations), Peepers (4 stations), and Toads (4 stations).
Treefrogs hadn't been heard since 28 June, and the resumed calling went on
until 24 July; Peepers hadn't been heard since 27 May, and weren't heard again
until fall calling began. Toads hadn't been heard within the area of the
circuit since 15 June, and weren't heard again.

Discussion: The reduced calling by Green and Bull
Frogs may have been due to the lower temperature on the 10th or it may have been due to males abandoning
calling for terrestrial foraging in the newly dampened surroundings of the
water bodies. This latter idea may be supported by the lack of decline in the
calling of the resolutely aquatic Mink Frogs, and the number of Green Frogs
found on the streets, 300 m from the nearest water, though none of the ones
that reached the village were adult males that would have been calling.

In normal springs, Treefrog calling is more or less
continuous between the calling at the breeding ponds, calling during dispersal
from the ponds, and calling from the trees of the summer habitat. There's more
calling in rainy summers, and one of their vernacular names is Rain Frog, but
there's also fluctuations in abundance, and understanding the meaning of their
summer calling will call for a detailed analysis. This year, after the rain,
calling tapered off during 11-12 July, with some also heard on 17 and 24 July
as the drought continued.

July calling by Toads isn't unknown, we've had it locally
in 2003, 2004, 2008, 2009, and 2013; these seem to be isolated males from the
shores of the creek or other water bodies, but we've never gone out and found
any of the July-calling males.

This is the first time we've had calling by Peepers in
early July -- all previous July records have been late in the month, a few days
before the traditional onset of fall calling on 1 August. In this case we have
to assume they were jolted into calling by the novel sensation of rain -- or
perhaps they were holed up partway along the routes to summer habitat, and were
calling as they they would have at those same locations in a normally wet
spring while dispersing from the breeding sites towards summer habitat.

The interesting thing about this unseasonable calling is
that, unless there were some very late-season female Toads, it's all
unassociated with breeding. The functions of autumnal calling are hard to study,
but the suggestions are either hormonal urges left over from, or preparing for
breeding, or else territorial spacing during the feeding season.

Climate change is supposed to increase the irregularity
and intensity of droughts, and we'll need both regular monitoring and careful analysis of the
gathered data to understand its effects on our Amphibians.

24h40-24h46.12°C,
clear, calm. Bull Frog, index2small
loud nearby chorus; Green Frog, index1 few calls; Mink Frog, index1
very few standard calls, but this species may have produced a few 'glumph'
and 'blurt' calls; Peeper, index1 very few calling.